The present invention relates to rotary knife holders in general, and more particularly to improvements in rotary holders for knives which can be used in apparatus for severing sheet stock, such as running webs or strips consisting of paper, cardboard, metallic foil, plastic foil or the like. Still more particularly, the invention relates to improvements in means for damping the oscillations of rotary knife holders.
Apparatus for severing running webs or strips of paper or the like utilize one or more rotary holders for knives which extend transversely of the path of the running web or webs and sever the web or webs once during each revolution of the holder or holders. In many instances, apparatus of the just outlined character (also known as transverse cutters) employ two rotary knife holders, one at each side of the path along which the web or webs are transported lengthwise. Each knife of one holder cooperates with a different knife of the other holder to sever the web or webs in the region where the web or webs pass through the nip of the two holders. It is also known to employ a stationary knife holder at one side and a rotary holder with one or more knives at the other side of the path of transport of one or more webs. If the apparatus uses two rotary holders, the peripheral speed of such holders (such rotate in opposite directions) matches the speed of forward movement of the web or webs (at least during the intervals of severing). Apparatus of such type are known as parallel transverse cutters.
The peripheral speed of the rotary holder or holders (i.e., the speed of the cutting edge or edges of one or more orbiting knives) varies in dependency on changes of the speed of lengthwise movement of the web or webs. Thus, the peripheral speed of the holder or holders increases with increasing forward speed of the material which is to be severed to yield sheets of desired length. As a rule, the manufacturer will wish to operate the severing apparatus at or close to the maximum capacity, especially if such apparatus forms part of a production line wherein wide webs are subdivided into narrower webs or strips which are thereupon subdivided into discrete sheets preparatory to stacking of such sheets and conversion of stacks into note books, steno pads, exercise books or like stationery products.
It has been found that, when the peripheral speed of the rotary knife holder or holders reaches a certain threshold value, the knife or knives are likely to "skip", i.e., they fail to sever the running web or webs. The failure can be complete (namely, the knife or knives do not sever any part of the web or webs) or partial (thus, the knives can score the web or webs or even partially sever the web or webs but the severing action does not suffice to form a series of discrete sheets). The so-called skipping can also amount to inferior cuts, i.e., the cuts across the running web or webs are complete but not clean so that the resulting discrete sheets exhibit ragged edges and cannot be properly stacked and/or otherwise processed.
Experiments with presently known transverse cutters indicate that the just discussed skipping is attributable to oscillation of the rotary knife holders. Thus, when the knife on a holder severs a web, the severing action is felt by the respective holder in the form of an impact which initiates oscillatory movements. If the peripheral speed of a knife holder is constant, the oscillatory movements are attributable to flexing of the holder, i.e., vibrations which are attributable to torsional stresses are negligible or non-existent. However, the torsional stresses are or can be sufficiently pronounced to cause readily detectable oscillations of the knife holder if the latter is driven at a variable speed. When the speed of the holder exceeds the aforementioned threshold value, oscillations which are attributable to torsional stresses and/or bending are sufficiently pronounced so as not to disappear during the interval between two successive cuts across the running web or webs. This compounds the problems and often or invariably causes skipping which can be discerned due to total absence of cuts or due to the making of inferior cuts. In other words, the positions of cooperating knives on two rotary holders or a rotary and a stationary holder can or will vary from severing action to severing action if the speed of the knife holder or holders reaches or exceeds the threshold value, and the unpredictability of positioning of the knife holder or holders during successive severing steps increases if the speed of the holder or holders is not constant. If the frequency and/or amplitude of oscillations is excessive, at least one of the holders is likely to move the cutting edge of its knife away from the cutting edge of the other knife at the exact moment when the knives are to sever the running web or webs with the aforementioned result that the current edges will fail to sever the web or webs or will perform an unsatisfactory severing action.
An analysis of the just discussed phenomena can be found in the German-language publication entitled "Messtechnische Briefe" 2 (1969), pages 35-39. The just mentioned article refers to the possibility of remedying the situation by regulating the behavior of knife holders but fails to furnish any concrete solutions.